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Specifier's choice:Roundhouse, London

Sutherland Lyall talks to John McAslan + Partners project director Adam Brown about the transformation of Camden's Roundhouse - and the benefits of a demanding client

The Roundhouse's modern era started in 1966, with a benefit concert for the underground newspaper International Times by Pink Floyd. The building, in Camden, north London, was a fire trap and had long been derelict, although Arnold Wesker had started a short-lived theatre there two years before. Despite its primitive and sometimes dangerous facilities, the sheer size of its internal space made it a natural venue for rock shows, theatre and dance productions.

The vast brick drum with a partly glazed conical roof was originally built as an engine-turning shed, perhaps also housing the winding gear that pulled underpowered steam trains up the hill from Euston Station half a mile to the south. In 1869 it was turned into a liquor warehouse for Gilbeys. Rot, abuse, time and an absence of imagination about its possible use meant it had mouldered quietly until the night of the concert in October 1966. Since then there have been many plans for its rejuvenation. There is an office development on the fringe of the site and a Safeway and its car park shoehorned in behind. But the venue itself was a bottomless pit for inspired thinking, grand dreams and inadequate funding, until millionaire Torquil Norman of toy company Bluebird bought it almost on a whim and began setting it up as a performance space and centre for young people.

The Foster practice carried out an early feasibility study, but it was John McAslan + Partners that won the commission in late 1997. By the end of 1998, the architect had produced a scheme, which Norman took to the Arts Council and the Heritage Lottery Fund. He was promptly turned down - largely on the grounds that it was overambitious. Undeterred, and having already put around £3 million into the scheme, Norman went out and raised promises of money from the Department for Education and Skills, the London Development Agency, English Heritage and the Wellcome Trust, and returned to the Lottery Fund. This time, impressed by the sums, the business plan and Norman's evident determination, the fund, via the Arts Council, came up with a substantial grant.

Around this time Norman was able to buy a triangle of land adjacent to the site from the Home Office. This allowed McAslan to pull virtually all the above-ground accommodation from the giant brick drum and locate it in a separate two-storey structure.

The new structure would follow the curve of the drum, but would be separated from it by a grand staircase in a high glazed-entrance atrium in the gap between old and new.

This left the vast 'basement' - actually at street level - for children's activities, including a performance space, internet radio station, multimedia suites, mixing and editing suites, practice rooms and a TV studio.

The wonderful vaulted spaces radiating from a central circular room then became the performance area. As the basement was merely the heavy brick base that raised the engine floor up to the same level as the railway lines on the other side of the site, these vaults were a consequence of the original builders being economical with building materials rather than architectural design.

In the great domed, Pantheon-esque space above, the ring of slender cast-iron columns and arching brackets seem so slender that they seem to only just support the roof.

At first-floor level is a continuous internal perimeter balcony, much narrower than the original, beneath which are dressing rooms and WCs. All the other essential spaces, the bar, administrative offices and a hospitality suite are in the new segment, connected to it by two steel bridges flying across the atrium from the balcony level to the second floor of the new wing. Project director Adam Brown says: 'We felt that the main space was so extraordinary that, if we could, we would have pulled everything out of it.'

This could have been the Roundhouse's last chance, so everybody involved wanted it to succeed. Brown says planning was very straightforward. 'We had an excellent relationship with Camden council and English Heritage. The only difficulty in terms of planning was parking provision. So we had to write a tough travel policy that involved discounts for people who came by Tube. The 16 parking spaces are largely for disabled people and a few late-night staff.' Negotiations have been going on with the adjacent Safeway store about renting spaces in its car park for evening audiences.

The JCT contract was in two phases. The first was with Willmott Dixon. Brown says: 'We worked with them for nine months. Then in summer 2003 the client felt the price wasn't as competitive as it could be. So it was retendered. Willmott Dixon felt rather aggrieved, but they were paid and there was provision for a retender in the contract.' In the event, Tolent Construction won the contract in April 2004 and started on site in June.

All the consultants are contracted directly to the client, with Anthony Hunt (now SKM Anthony Hunt) as the structural engineer and Buro Happold as the services engineer.

The client has a project manager. In fact, there have been several. After a brief interlude with a specialist, Davis Langdon was appointed, bowing out at around stage C.

Tony Hudson then took it through to the first-stage tender, before leaving to run the Battersea power station project. 'There was briefly another chap, ' Brown says, 'and we now have a team, Applied Solutions, that is really quite contract oriented.' The chief executive, Marcus Davey, came from the Brighton Festival in 2000 and, together with Norman, works as the client.

This is a JCT contract with contractor design portions and a single nominated subcontractor, Slingco, which makes the cable tension net used for access to the main theatre grid. This is like an open trampoline, which is effectively transparent for the theatre lighting. The contractor design packets are for glazing and cladding, and some elements of the roof. The latter is slate on a new steel-lattice structure, with the original ring of glazing and the glass 'umbrella' at the apex, with sound insulation. 'The roof has complex interfaces and we felt that the design coordination responsibility should stay with the contractor, ' says Brown.

Brown is philosophical about the profitability of the project. 'On a project like this, you begin the process in the knowledge that it is not going to be profit-making. We just wanted to do it, ' he says.

Brown has used the National Building Specification and seems to have had to justify most choices of material. He says: 'To be fair to the client team, the cost-driven stuff has been in stopping the new building from getting too big. The clients wanted to know what everything is made from and whether it's tough enough and of a reasonable quality. We've had our disagreements, but they've been receptive. They've challenged us on durability and, although Norman is not aesthetically driven, he's good at spotting when things aren't quite right.'

Brown says of his design intentions: 'We tried to keep everything simple, because the brickwork in the main building is so rich. In the new wing everything is relatively light and pale: Yorkstone and pale-grey resin on the floors, reconstituted stone on the main stairway, and painted plaster walls. We were keen that passers-by could see activity in the bars as a counterpoint to the introverted quality of the great brick drum, so there is to be a shift between old and new. In the drum we have used a darker palette of dark, brick-red panelling and graphite metalwork - this suits the theatre people, who like the colours to be recessive so the audience can concentrate on the performers. In theory, the colours get paler as you move away from the middle of the old building.'

Brown's team looked first at the acoustic performance of walling materials and decided on a perforated particle-board panel with a rockwool backing and a horizontally ribbed face - with ribs around 12mm apart. It is to be coloured a dark, rusty orange in the basement areas and a deep grey in the performance space. The panels will be 900mm wide by 2.5m to 3.5m high, depending on their location. Ceiling-height doors are faced with the same material, so that they look like panels when they are not in use.

Floors in the old building are almost always an epoxy Isocrete screed, although in some areas the concrete is more conventionally power floated. 'We never really considered using a sheet material, ' says Brown. The theatre spaces will have timber floors. In the new wing, the majority of floor surfaces are a resin-based screed from German firm Bollid. This is a pale-grey fleck around 4mm thick, and is poured over the slab screeds.

The offices are carpeted, while in the atrium the Yorkstone paving matches with surviving Yorkstone paving on the footpath outside.

Brown's team had toyed with the idea of a steel frame for the new wing with blockwork infill, but he says: 'The contractor, Knight Build, said it would be easier to use a standard curved formwork. The tricky bit has been getting the quality. We showed them what we thought was good quality and they showed us four or five samples of work they had done.' The curved outside face of the new wing is to be clad in louvred dark-grey anodised aluminium panels, each 1.3m wide by 6-7m high.

Brown explains the choice: 'There are quite a few services visible on the outside wall: grilles, inlets, outlets and the like. So the skin with louvres at 45° is fixed 100mm outside cobaltblue painted Rockwool insulation bonded to the concrete. This assemblage is from English Architectural Glass (EAG), which is also responsible for double-glazing - including the glass atrium roof - and the complicated glazing on the conical roof of the drum. EAG was one of the few firms prepared to tender for the roof, while for the even more complicated steelwork, William Hare was one of only three firms that the structural engineer said should be on the tender list.

Architectural steelwork, including the bridges and stairs, walkway and balcony balustrades are by Delta, a firm suggested by the contractor. Prior to the tender stage, Brown had worked with John Desmond, but he says philosophically: 'With largish projects where the commercial scale is acute, you have to sit back and work with the contractor's choice.

In fact, Delta has been helpful - it's making mock-ups now and booking the work to go in later in the day.'

Brown specified Allgood D Line ironmongery and has managed to hang on to it. 'We haven't even had a fight about it, ' he says. 'I don't know why it hasn't fallen foul of all the value engineering. I guess the price was keen. Allgood's products are really elegant and tough, and they're very helpful with the technical stuff, which can be labyrinthine.

They're also very clever. In the music practice rooms there are hotel-like swipe entry cards - with children you have to have this kind of security - which are really neat. They're simple rectangular plates that you touch the card against. And the cards are plastic and cheap to replace, unlike other card systems.' The ceramic hand basins were specified from 'a nice range designed by David Chipperfield'. They are D-shaped in plan, with circular bowls. Because they will be used by rock concert-goers they have to be bolted directly to the walls.

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